A variety of implantable medical devices for delivering a therapy and/or monitoring a physiological condition have been clinically implanted or proposed for clinical implantation in patients. Implantable medical devices may deliver electrical stimulation or drug therapy to, and/or monitor conditions associated with, the heart, muscle, nerve, brain, stomach or other organs or tissue, as examples. Some implantable medical devices may employ one or more elongated electrical leads carrying stimulation electrodes, sense electrodes, and/or other sensors. Implantable medical leads may be configured to allow electrodes or other sensors to be positioned at desired locations for delivery of stimulation or sensing. For example, electrodes or sensors may be carried at a distal portion of a lead. A proximal portion of the lead may be coupled to an implantable medical device housing, which may contain circuitry such as stimulation generation and/or sensing circuitry. Other implantable medical devices may employ one or more catheters through which the devices deliver a therapeutic fluid to a target site within a patient.
Implantable medical devices may include one or more physiological sensors, which may be used in conjunction with the device to provide signals related to various physiological conditions from which a patient state or the need for a therapy can be assessed. Examples of such implantable medical devices include heart monitors, pacemakers, implantable cardioverter defibrillators (ICDs), myostimulators, neurostimulators, therapeutic fluid delivery devices, insulin pumps, and glucose monitors.
Optical sensors may be employed in implantable medical devices as physiological sensors configured to detect changes in light modulation by a body fluid or tissue volume caused by a change in a physiological condition in the body fluid or tissue. Such optical sensors can be used, for example, for detecting changes in metabolite levels in the blood, such as oxygen saturation levels or glucose level, or changes in tissue perfusion. Monitoring such physiological conditions provides useful diagnostic measures, and can be used in managing therapies for treating a medical condition. For example, a decrease in blood oxygen saturation or in tissue perfusion may be associated with insufficient cardiac output or respiratory function. Thus monitoring such signals allows an implantable medical device to respond to a decrease in oxygen saturation or tissue perfusion, for example by delivering electrical stimulation therapies to the heart to restore a normal hemodynamic function.